1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device, and relates to, for example, an arrangement of wells of a static random access memory (SRAM) and a flip-flop circuit.
2. Description of the Related Art
Recently, as a semiconductor integrated circuit devices is formed more densely, a power supply voltage is being reduced to nearly 1.0 V. Accordingly, a data retaining ability of SRAM becomes a problem. SRAM retains voltage information, and therefore, a slight change in a voltage due to noises or the like may easily destroy retained data under small power supply voltage. As a result, a problem of defective SRAM is being highlighted.
Particularly, a soft error is getting serious enough to restrict shipping of SRAM products due to its reliability problem. The soft error is a phenomenon such that fine particles of α rays, neutron radiation and the like butt against elements, its energy forms electron/positive hole pairs, and these electric charges destroy data retained by the elements.
As a countermeasure in a form of circuits against the problem of destruction of data, mounting of an error correct circuit (ECC) is being mainstream. The ECC checks a parity bit added to cell data of a memory so as to automatically find destroyed cell data and restore them. The countermeasure using the ECC is effective for the case of rather small destruction of the cell data.
Even the soft error countermeasure using the ECC, however, cannot cope with some cases because of the following reason. At present, as shown in FIG. 1 of Jpn. Pat. Appln. KOKAI Publication No. 2003-100904, linear n-type wells NWEL and p-type wells PWEL are formed in order to facilitate the integration of memory cells. Transistors composing the memory cells are formed in the wells NWEL and PWEL. Therefore, the memory cells which are adjacent to each other in a direction in which the wells NWEL and PWEL extend share the same well. For this reason, it is strongly possible that electrons and positive holes generated by butting of fine particles of α rays, neutron radiation and the like propagate through the wells NWEL and PWEL so as to influence the other transistors. Because the chain reaction simultaneously destroys a lot of cell data, even ECC cannot restore all cell data. For this reason, the soft error countermeasure using the ECC cannot be perfect.